Sample size for observational study

Hi everyone!

I should to calculate sample size for observational study in that the primary endpoint is to assess the distribution of COPD by 4 classess (COLD). According to article i know expected distribution by class:
1 class: 16%
2 class: 25%
3 class: 28%
4 class: 31%

How I may to estimate the sample size for this study? The population size is 1000000

Many thanks!